This is a very simple mod that anyone with a spanner & common sense can do easily themselves. Really, no mechanical skills required & the cost was AUD$6. I've included photos to walk learners through the process & be confident they've done it correctly. If you know what you're doing, you could tweak it for different sounds, but I will leave that to you. AFRON1, who knows more than me, has some good ideas on this below if you want a more sophisticated solution. I don't, 6 bucks was my budget, so I'll just explain the basic process. (Wikipedia's explanation of what the blow-off valve does it at the bottom). Once done, your Copen will put any WRX to shame when it comes to blow-off wars! Just back off the throttle, eg shifting gears, and you will be amazed at the "whoosh!" noise it makes. I find it a very addictive sound but I get that some owners wouldn't like it. I also have a direct cone air intake & a straight-through 2"exhaust with a 30cm length of 2.5" for some echo. But nothing mad, she's otherwise stock except for the real time engine temp sensor & display. I never even look at the temp gauge on the dash now, it's a very unhelpful thing. Be it 50deg or 90deg, it just sits in the middle. Mine's set to go off at 100deg, as we get 45deg days here in Canberra. BACKSTORY: Recently my Copen came back from her 60k logbook service, sounding very different. Every time I took my foot of the throttle there was a loud "whoosh!" sound coming from the engine bay. I liked the sound but went back to ask what was different, as different is usually bad. Luckily, the manager where my car is serviced is a Daihatsu turbo enthusiast! He has just installed a 1.6L turbo (he spent 4 yrs finding the half-cut) into his 1986 Charade & now it pulls 128kW on the dyno! Insane! But a very lucky co-incidence! Anyway, an apprentice had simply not re-connected the pipe between the blow-off valve & the re-direct to turbo. Instead, it was venting to atmosphere. The manager explained to me the plumbing from the blow-off valve & how the excess pressure is re-directed to the turbo via this pipe. I said "Ï like the sound. How do I do this properly & is it potentially harmful to my engine?" He explained that it is not harmful at all, as long as the pipe leading to the turbo is properly covered to protect from anything being sucked in there. It does not have to be air-tight, just firmly secured. So, effectively, all you have to do is remove the plastic pipe that re-directs to the turbo & install an off the shelf metal cap with a metal hose clamp over the turbo intake pipe. I bought a 38mm metal cap at a metal shop for 70c & the hose clamp for $5. Half an hour later, I was done. The hose clamp can't really squeeze the strong metal clamp more than maybe 0.5mm but the 38mm cap fits snugly anyway. I used some contact adhesive & tucked the tightening bit of the clamp under a convenient engine hard point just to be sure. MECHANIC'S APPROVAL: I went back to my mechanic to ensure I had done the work properly & he signed off on it as "just how I would have done it." WARNINGS: 1. Do not use silicone as it will harden over time & bits may fall into the turbo spinning at 120k RPM! This will be very detrimental to your turbo! 2. Also, be very careful not to stab yourself with a screwdriver as you struggle to remove Daihatsu's original hose clamps! PROS: That fantastic turbo "whoosh!" every time you take your foot off the throttle! A very easy & cheap mod, no mechanical skill needed, just be careful to cap properly Does not stress the engine CONS: Very marginal loss of torque below 3k RPM (not even noticeable). (Relevant equation: "Bugger-all torque -1 = Bugger-all torque" If you don't have a real-time engine temp sensor & display fitted, and I highly recommend you do, it may affect your dash temp gauge a little bit I was told. I'd love any feedback on this from whoever reads it. Cheers from Canberra, Brian! FROM WIKIPEDIA: A compressor bypass valve (CBV), also known as a blow-off or diverter valve, is a manifold vacuum-actuated valve designed to release pressure in the intake system of a turbocharged vehicle when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurised end of the intake (before the turbo). This mod instead vents it to atmosphere.